Cavity antenna



Nov. 3, 1953 A. DORNE ETAL CAVITY ANTENNA 2 Sheets-Sheet 1 Filed Jan. 7, 1946 INVENTORS ARTHUR DORNE By DAVID LAZARUS .%k...9./AM.

ATTORNEY NOV. 3, 1953 DORNE EIAL 2,658,145

CAVITY ANTENNA Filed Jan. 7, 1946 I 2 Sheets-Sheet 2 FIG. 3

-FIG.5 FIG.4

20 70 20 1 l L v 40\ Lg'wfi A50 INVENTORS.

ATTORNEY Patented Nov. 3, 1953 CAVITY ANTENNA Arthur Dome and David Lazarus, Cambridge,

Mass., assignors to the United States of America as represented by the Secretary of War Application January 7, 1946, Serial No. 639,653

18 Claims.

This invention relates generally to electrical apparatus. More particularly it relates to flush mounted radio antennas particularly suitable to aircraft installation.

In the installation of radio equipment on aircraft it is desirable to employ antennas which offer a minimum of wind resistance and contribute a minimum of drag; which are as compact as possible; which offer suitable directional characteristics; and which, for certain adaptations, are broad band, that is not excessively sensitive to frequency changes. There have been several methods used to meet these requirements. Antennas which have been mounted so as to extend outside of the skin of aircraft have been formed to have contours adapted to minimize wind resistance; their physical size has been kept small by the selection of high operating frequencies as well as by the use of designs resulting in certain electrical dimensions being greater than physical ones; and broad banding has been achieved by the use of radiating elements having relatively great thickness and by other design features well known to the art for producing frequency insensitivity. The conventional method for reducing wind resistance, which is described above, is reasonably satisfactory for aircraft which travel at low and moderate speeds. However, in very high speed aircraft, antennas which extend beyond the skin of the aircraft, even if they are streamlined, contribute substantial amounts of drag.

It is an object of this invention to provide a radio antenna which can be mounted flush with the skin of aircraft so as to present no appreciable protrusion from the exterior surface of the craft. It is another object to provide an antenna which will have small physical proportions thus requiring little space inside of the plane, and it is a further object to provide an antenna which will have satisfactory directional and broad band electrical characteristics.

Generally this invention comprises a cavity having a slot-like aperture somewhat resembling in appearance a short section of wave guide closed at one end which acts as a radiating source for radio frequency energy. This cavity is excited internally by a radiating element mounted within it and is excited in such a way that an electromagnetic field will extend out of its slotted or open end in a suitable radiation pattern. The open end or slot of said cavity may be adapted for flush mounting on the surface of aircraft, and said cavity should include design features and elements rendering it both compact and with a broad operating-frequency band width.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

Fig. 1 is an isometric view of a simple embodiment of a slot antenna cut away to show the manner in which the exciting element is connected to the cavity portion and also to show the pressure of an end plate closing the side located opposite to the slot.

Fig. 2 is a view taken directly into the open end of another embodiment of the slot antenna showing the manner of installation of a matching device which is a physical extension of the exciting element and which gives it the general conformation of a letter T.

Fig. 3 is an isometric view of another embodiment of a slot antenna which is capacitance loaded by distortion of the shape of the cavity, and this figure, as does Fig. 2, shows a T-shaped exciter.

Fig. 4 is a sectional view of the embodiment shown in Fig. 3 with the addition of a dielectric block element which serves as an additional means of capacitance loading, the sectional view being taken in a plane passing through the center of the T exciter.

Fig. 5 is a view taken into the open end of an embodiment similar to that shown in Fig. 3 but which employs an alternate means of matching the exciter to the slot.

Referring now more particularly to Fig. 1, element la is a cavity having the same physical characteristics as a short section of metallic Wave guide. It has opposite sides 2a and 3a across which a radio frequency electric field will exist in the intended mode of excitation. It has two other sides 4a and 5a which are also opposite to each other and which are joined to sides 2a and 3a to form a conductive tube having a substantially rectangular cross-section. This tube is open at one end, and it is closed at the opposite end by end plate 6a which may be an integral part of cavity la. It is because of the appearance of the open end that the name, slot antenna, has been applied to this device. There is fastened to side 2a a connector m by which a lead-in cable coming from the radio frequency power source can be attached. This connector may be of conventional design and may include within it a coaxial or other matching transformer adapted to match the input impedance presented by the radiating element or exciting probe 8a to the output impedance of the lead-in line.

- ment which does'no't employ In part radiating element Ba. radiates directly out of the open end of the cavity la. However, it also radiates energy in the opposite direction, 1. e. toward end plate 6a. This energy, however, is reflected from end plate fia and, therefore, it, also, is eini'tted f fb' the open s1q The jdistance between *erid plate Ga and radiating element 8a and the distance between the both play a part in .matching theantennajo the load presented byspaceu Radiating element fla should be mounted back frorn the $101; by a distanceflof less than one-eighth of fa wavelength plus any integral number of half The wavelength used as the basis of this Measurement is that which correspondseto the lowest intended operating frequency. The distance be tween element 8a and the side opposite to the slot, as measured in terms of the same wavelength, should be more than'one eig'hth and less than one qnarter of a wavelength plus any in- "tegral'number er half wavelengths. v

Heferring new to Fig. 2, it will be seen that this "embodiment of the slot antenna employs a radiating element 81) which is s1ightly 'difi'e'r 'ent from the corr sponding element "13a of Fig. *1. This slight difference in its shape results from the fact that element 8b is adapted to be 'slttaclied to additional element 9b. Also asseerr-tee with elements 8b "and 9b is a dielectric sheath element lllb which is a covering surshunning element '91). Element 'H'lb in'ay 'or may nb'tbe employed depending on the requirements of the particular installation. Element 91) has substantially the shape of a cylindrical bar of cdnducti've material about equal "to the diameter of radiating element 81). It is physically attached to eleme t 8b and serves to connect it at a corresponding to what is the "free end in Fig. 1 to "sides "41) jand b of 11). It has been found that, by using a conductive element such as 9b "and by iziirine'ctii'ig it in this way, matching of the antenne to the radio frequency source can be achieve withcut reliance upon the use of a "large dimension, in terms of one of the wavelfi'gtlis described above, as measured between radiating element 81 and the end plate of the "cavity 'llb. "Instead, matching is 'eifected bythe presence of element 3b as well'a's by the length dir'nerfsion "of cavity l'b. Element l 0b hiakes it possible for element sb'to have the effective-e ectr1cal length nec ssary for 'it to perform such a matching function without sole reliance on its *pliysical length.

fof'element ta cavity element The manner in which element lflb accomplishesthis and the 'manner in which element 9'!) broadens the operatin band Width 'will-be disclosedmore fully hereinafter, Referring now to Fig. 3, it will be seen that sidszc and 3c of cavity 1c are distorted in the igion of their central portions'so as to approximate one another more'clos'ely. This'disturbs "the sinusoidal symmetry of 'theel'ectrical field strength distribution between sidesiic "and 30 and, therefore, 'capacitively loads the slot an- 'te nfa. 'It'h'asbee'n found thatfwhere capaci- 'jtan'c'e loa'ding'is employed it is possible to reffdu'c'e 'the'other dimension 'of the cross-section pf cavity Ic without reducing the cutoff fref qi ,nQ'yIOn the long'wavelength side of the p- :erating'frequency band. Therefore, inthis en' 'bodiment, the dimension measured between 111 1; n g qi mb mal 'p s lr ha he "cbrfespofiding l em ns p hman. m psl capacitance inauing. Because of the contemplated use in airslot and element 8g "in effect,

4 craft, this more compact construction without sacrifice of important electrical characteristics, in particular the ability to function at relatively long wavelengths without cutoif, is most desirable, and it meets one of the objects of the invention v a Referring now to Fig. '4, it will be seen that a dielectric block ll occupies the space between sides 20 and 3c in the region where sides 2c and 3c are made to approximate each other. llt can be readily understood by the application of fundamental principles that the pres- 'nee b'f dilctric block II also capacitively loads "a; Because of this, the use of b lgck H -perrn its a;further reduction in physi ca1 si z e, particularly a reduction of the dimeneaten measured between sides 40 and 50. All of the space inside of cavity Ic can be similarly filled with any suitable dielectric substance, "such as polystyrene, and the overall effect is to per;- mit even smaller physical dimensions. Several theories have been advanced asto why this is so. These "explanations relate either to the concept of capacitance'ioadin'g or to the well known concept of the effect on the electrical dimensions of cavities and guides which results from "a reduction 'of the velocity of propagation. Whether these theories are correct or not, it has been found empirically that a physically smaller slot antenna. is the result of this type of construction.

Referring now to Fig. 5, it will be seen that no matching element of the. type used in the embodiments shown in Figs.v 2, '3, and '4 is used. Instead, in this embodiment, that type is replaced by a transmission line matching stub of selected length connected to the end ofradiat- 'ing element -8d which corresponds to the free end of element 811. in Fig. 1. In the embodiment shown in'Fig. 5 this stub consists of pigtail 12 of cda'xia'l transmission line which is short circuitedat one end and is connected *at its :other end to the end of radiating element "8d described "above. The length of this pigtail can'b'e'selected so that it ofiers a lumped value of capacitance and balances out "the inductive react'ance presented-by th'e'end plate. The foregoing portion of this disclosure has described the physical construction and arrangement of several embodiments of this invention and has touchedupon'certain of the-principles of operation. The portion which follows immediately hereafter is intended primarily to explain somewhat-more fully the-principles of operation.

The us of amatching element, of 'the type shown in Fig'sx2, *3 "and '4' as'elements 9b 'andSc,

"offefs an advantage in one particular feature er its ne rorhrenee.

It causesthe antenna to "a greatly increased "frequency band. This is due to the fact that this type of matching *erein'ent and the 'end plate of ma bothi ofier reactanc'es which are H U one fcohc'eptfof an equivalentcir- 'cuit of this antenna, interposed in series with the-tam t ieeu presented by the 'antenna'and that in the 'sugge'sted arrangement they tend to can- I this matching element is not used and if" 'nste'ad the reactan'ce shared "by thejendplateisf djusted to a value suitable fQ1f IIl B.1;0hlI1g "by appropriate selection of "the distance between it and the radiating element, this 'selection ould be correct only at or near "a specific. frequency. I- I'qwever, where tlie matching "element -15 need not ni uses n "i5:- fer compensation to the reactance oflered by the end plate so that the distance described above can be a short one, with respect to one wavelength corresponding to the operating frequency. particularly on the long wavelength end of the operating range, but, in addition, this compensation is effective over a realtively wide frequency band. This is due to the fact that throughout this relatively wide frequency band the reactances offered by these elements have independent tendencies to change somewhat equally as frequency changes occur, and the changes, of course, are opposite. In general the reactance offered by the matching section is capacitive while that offered by the end plate of the cavity, particularly when the guide is very short in length, is inductive.

The manner in which this type of matching element presents capacitive reactance to the free end of the radiating element by being electrically connected between that free end and the sides of the cavity which have been described as do. and a; 4b and 5b; 4c and 5c; and 4d and 5d can be explained as follows:

The matching element itself may be looked upon as the inner conductor of a coaxial line while the hollow space formed by the cavity element of the slot antenna may be regarded as the outer conductor. The axis of said coaxial line would, of course, be along the axis of the matching element. When so considered, it immediately becomes obvious, by the application of well known transmission line principles, that its electrical length determines the reactance reflected to the feed point. If the electrical length is between one-quarter and one-half wavelength, this reactance will be capacitive. Where the dimension requirements of the cavity require it, this section of line can be made electrically longer than its physical length by the expedient of filling up the space between the inner and outer conductors with a dielectric material. This effeet is achieved through the use of a dielectric sheath of the type shown in Figs. 2 and 4 as elements I92) and Ito. The presence of a dielectric having a dielectric constant greater than 1 results in a reduction of the velocity of propagation along such a line, and, therefore, an electrical Wavelength occurs in a physical distance less than a wavelength. It will be noted by reference to Fig. 2 that the dielectric sheath need not fill up all the space between the outside of the matching element and the inside of the cavity, but can be made of thin-walled material. Because of the fact that the lines of electric force about the matching element converge upon it and have their greatest intensity near to it, the dielectric sheath offers its greatest effectiveness near to the matching element and isnt essential in the remaining space. Of course, there is no reason why a sheath of greater thickness cannot be used if desired.

It hasv been found that in slot antennas the operating band width is reduced when sides corresponding to sides 2a and 3a of Fig. 1 are brought closer together. This is also true where those sides are distorted in the manner explained above with respect to the embodiment shown in Fig. 3. Conversely this type of antenna becomes more broad banded when said sides are placed further apart. Therefore, in the use of antennas of this type considerable latitude in giving preference to band width or compactness or vice versa, for satisfying specific requirements, is available.

Where narrowing of the band width results from capacitance loading, it may be found desirable to broaden the band width by the use of a- T-matching element such as that described above and numbered 9b and 90. In applications, in: which the distance across the cavity where such a matching element should be located is reduced to the point where the matching element cannotbe long enough to offer the proper reactance, a transmission line matching element, such as element i2, may be employed.

It will be obvious to those skilled in the artthat there is a wide latitude of choice in the actual details of construction of slot antennas employing the principles covering this invention. There is a wide latitude in the choice of the proportions of the slot and of the various dimensions of the cavity. The physical distortion used to capacitance load the cavity can take a variety of forms, and a wide variety of dielectric materials can be employed to achieve the same end. It is also obvious that flanges or other suitable mechanical supporting means might be added to slot antennas, preferably at their open ends, adapting them to flush mounting on the skin of aircraft. Likewise cover means can be used to keep out foreign matter such as water, ice and dirt. Fibre glass has been found to be a suitable material for this purpose and does not interfere objectionably with the radiation patern.

It will be seen that the slot antenna herein described will provide a flush form of installation contributing no drag whatsoever in aircraft; that it can be constructed in very compact dimensions; that it can be made to be a broad band device; and that it has suitable directional characteristics, in particular, directivity away from the aircraft and/or toward the ground.

It will be apparent that there may be deviations from the invention as described which still fall fairly within the spirit and scope of the inven tion. Accordingly we claim all such deviations which fall fairly within the spirit and scope of the invention as identified in the hereinafter appended claims.

What is claimed is:

1. A slot antenna including a cavity, said cavity having substantially rectangular -cross-sections, a pair of opposite sides of said cavity being distorted so that the corresponding cross-section is not of exact rectangular shape and so that said pair of opposite sides are brought closer together near their centers, said cavity having conductive inner surfaces and one side which does not have a conductive inner surface, an exciter probe mounted so as to protrude into the interior of said cavity, said probe being mounted back from said side which does not have a conductive inner surface by a distance of less than one-half of a wavelength plus any integral number of half wavelengths, said wavelength being the one which corresponds to the lowest intended operating frequency, said probe being mounted ahead of the side opposite to said side which does not have a conductive inner surface by a distance of more than one-eighth and less than one-quarter of one of said wavelengths plus any integral number of halves of said wavelengths, means for feeding radio frequency energy to said exciter probe and transmission line stub matching means electrically connected near an end of said exciter probe.

2. A slot antenna including a cavity, said cavity having substantially rectangular cross-sections, a pair of the opposite sides of said cavity being distorted so as to vary the corresponding crosssection from its rectangular shape, and so that on see gether 'ar mieeemer'ssaia "avity sewage- 1 ductiv inner siirfaces' and one side is open, an exc ter rose madman s6 t prqniaeinte the interior of said cavity, a probe being mounted back from said open 'side bya distance of at least one-eighth of a wavelength P1 any integral number of half ayeierggtns, said wave length being the one which correspondstothe lowest intended operating frequency, saidpi'obe being mounted ahead of the side opposite to sa id openside by a, distance of more than one-eighth and less than oneuarter of one of said wavelengths plus any integralnuniberof halves of said wavelengths, for feeding liaalfi fref q'ue'ncy energy to said eiiciter probe, and transmission line stub matching means electrically connected near an end (if said exciter probe} v 3'. A slot antenna incliiding a cavity havin substantially rectangiilar c'rtisssections, census tive inner surfaces one side which does not have a conductive inner surface, an excitei' probe mounted so as to' protriide inns the interior or said cavity, means for f "edin'g radio frequency energy to said exciter probe, a conductive eler'nent inside said cavity, said conductive eriient being at tachd at its approximate center to an end of said excit'er piste and being arranged at susstaansny arig'ht angle to the axis of said probe, said conductive elenien beingarr'anged parallel to" said one side and electrically connected to a pairof opposite sides, the electrical length of said conductive element as measured from its cease-a ties as said probe to its connections re either or said opposite sides" bei more than cheque-Mr and less than one-halfof one wavelength corresponding to the lowest intended operating frequen y, and having a dielectric sheath-like eie: inent surrounding said conductive element.

4. A slot antenna including a cavity having substantially rectan lar cross-sections, condu'ctive inner surfaces and one open side, an e'jicitr probe mounted so as to protrude into the terior of said cavity, means for feeding radio frequency energy to said exciter probe, a conductive element attached at its approidmate centr'to an; end of said excited probe and being ar'ranged at substantially a right angle to the axis of sa d probe, said conductive element being arranged parallel to said-open sideandelectrically con-1 ne'cted to a pair of oppcsitesides', the'el'ectri'ca'l length of said conductive element as measured from its connection to said probe to its con-f nections to either of said opposite sides being more than one quarter and less than one-half of one wavelengthcorresponding tothe lowest in: tended operating frequency',and having a dielec said pair or opiaesite sides are tree tric sheath-like elem ntsurrounding said con ductive element. r

5. A slot antenna including a cavity having substantially rectan "lar cross-sections, ccnduc tive inner surfaces and one side which is open; an exciter probe mounted so as tojprotrude into the interior of said cavity, said probe being mounted back from said open sideb'ya distance of less than one-eighth of a wavelength plus any integral number of half wavelengths said wave length being the one which corresponds toth'e lowest intended operating frequency, said probe being mounted ahead of the side opposite to said open sideby a distance ofmore than onc eighth-and less thanone-quarter of'one of said wavelengths plus any integral number of halves of said wavelengths; a conductive eli l f flilt cllif heel-Led to" saidprobe anew-two opposite'conduF "gasses u ean r r eeamg 'radic netiienby ne' syte ai'dexcit h p a-i a s fifii in e ine d nl ac vity hav n substfnt 11y e an-gum cross sections, conductive i ner surfaces and one side which oe not have acondu'ctive inner surface, an exciter probe indunted s o as to 'p'rotrude into the interiorpf said mvnynte for feeding radid frequency energytfo saidprob'e, a conductive element within the cavity attached at its apprcximate center it ee b id i rq 'i an arranged a b-' s'tantiallya right angle to the axi ersaia probe, said esauaiveaemen being parallel to said one side and having a p es electrically connected to a pair of opposite sides having conductive inner surfaces, the electrical length of said ccndnctiy e elfinent as measured from its connection to said probe to its connections to either of said opposite sides being more than onegi iarter less than one-half of one wavelength correspdnding td the lowest intended operating new-Y. s. l v, l-

7. A slft, antenna including a cavity having substantially rectangular cross-sections, conductive inner surfaces and one open side, an exciter probe mounted. so as to protrude into the inte'rior ofsaid cavity, means for feeding radio frequency energy tosa'id probe, a conductive element within the cavity attached at its approximate center to an end of said probe and arranged at substantially a right angle to the axis of said probe, said conductive element being arranged substantially parallel to said open side and having its ends electrically connected to a pair of opposite sides, having conducting inner surfaces, the electri'cal length of said conductive elementasmeasured from its connection to said probe to its connections to either of said sides being more than one-quarter and less than one half of one wavelength corresponding to the lowest intended operating frequency.

B. A slot antenna comprising a cavity having substantially rectangular cross sections, one nonconductive side, the remaining sides being conductive', an exciter probe in the cavity, and a conductive cylindrical ,bar connected to said probeand to two' opposite conductive sides.

v 9} Aslot antenna according to claim 8 in whici a' portion' of the cavity contains an insulating material having a dielectric constant greater than that of air,

10. A slot antenna comprising a hollow rectangular parallelepiped in which five of the sides comprise conductive inner surfaces, an exciter probe mounted within the parallelepiper, and a straight conductive cylindrical bar connected to said probe and to two opposite conductive surfaces. I 1

ll. broad band antenna comprising a cavity with an H-shlaped cross-section, one of whose ii -shaped faces is non-conductive, the remaining faces comprising conductive surfaces, an exciter probe mounted on one of the conducting surfaces nd protruding into said cavity.

1;2. An1 ante a according to claim 11 further comprising a conductive element whose midpoint is connected to said probe.

13. An antenna according to claim 12 in which said conductive element is arranged parallel to the non-conductive face.

14. An antenna according to claim 13 in which the extremities of aid conductive element are electrically connected to opposite conducting faces of said cavity,

1'5I'A slot antenna including a cavity having substantially rectangular cross-sections, conductive inner surfaces and one side which does not have a conductive inner surface, a pair of 0pposite sides having conductive inner surfaces being distorted so that they are brought closer together near their centers, an exciter probe mounted so as to protrude into the interior of said cavity, said probe being mounted back from said one side by a distance of less than one-eighth of a wavelength plus any integral number of half wavelengths, said wavelength being the one which corresponds to the lowest intended operating frequency, said probe being mounted ahead of the side opposite to said one side by a distance of more than one-eighth and less than onequarter of one of said wavelengths plus any integral number of halves of said wavelengths and means for feeding radio frequency energy to said probe.

16. A slot antenna including a cavity having substantially regular cross-sections, conductive inner surfaces and one side which is open, at least a pair of opposite sides having conductive inner surfaces being distorted so that they are brought closer together near their centers, an exciter probe mounted so as to protrude into the interior of said cavity, said probe being mounted back from said open side by a distance of less than one-eighth of a wavelength plus any integral number of half wavelengths, said wavelength being the one which corresponds to the lowest intended operating frequency, said probe being mounted ahead of the side opposite to said open side by a distance of more than one-eighth and less than one-quarter of one of said wave- 10 lengths plus any integral number of halves of said wavelengths and means for feeding radio frequency energy to said exciter probe.

17. A slot antenna comprising a cavity having substantially rectangular cross sections, one nonconductive side, the remaining sides being conductive, an exciter probe in the cavity, and a c0nductive element connected to said probe and to two opposite conductive sides, said conductive element being surrounded by a dielectric sheath.

18. A slot antenna comprising a hollow rectangular parallelepiped in which five of the sides comprise conductive inner surfaces and the sixth side is non-conducting, an exciter probe mounted within the parallelepiped, and a conductive element surrounded by a dielectric sheath, said conductive element being connected to said probe and to two oppositely conductive surfaces.

ARTHUR DORNE. DAVID LAZARUS.

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